Calibration of a Terrestrial Laser Scanner - Institute of Geodesy and ...

16 2. Components of Terrestrial Laser Scanner2.1.6 Overview of Distance Measurement Techniques in Terrestrial Laser ScannersTable 2.2 gives an overview of the distance techniquesthat are used in terrestrial laser scanners. The corre¬lation between the distance measurement technique on one hand and the range and data acquisition rateon the other can be seen.Table 2.2:Overview of distance measurement techniques in terrestrial laser scanners. The data acquisition rate, i.e.frequency, and the range depend on the distance measurement technique.Laser Scanner Manufacturer Distance Technique Range[m] Frequency [kHz]CPW 8000 Callidus direct TOF 80 28LS 880 FARO AMCW 76 120HDS 3000 Leica Geosystems direct TOF 300 1.8LMS-Z420i Riegl direct TOF 800 12GX Trimble direct TOF 350 5Imager 5003 Zoller+Frohlich AMCW 53 6252.1.7 Avalanche Photo Diode (APD)An avalanche photodiode (APD) is a photodiode that internally amplifies the photocurrent by an avalancheprocess. A large reverse-bias voltage, typically over 100 volts, is applied across the active region.This volt¬age causes the electrons initially generated by the incident photons to accelerate as they move through theAPD active region. As these electrons collide with other electrons in the semiconductor material, they causea fraction of them to become part of the photocurrent. This process is known as avalanche multiplication.Avalanche multiplicationcontinues to occur until the electrons move out of the active area of the APD.Figure 2.10: Layout of silicon photodiodes: conventional design (left) and pin layout (right) according to [Meschede,2004].The gain of the APD can be changed by changing the reverse-bias voltage.A larger reverse-bias voltageresults in a larger gain. However, a larger reverse-bias voltagealso results in increased noise levels. Excessnoise resulting from the avalanche multiplication process places a limit on the useful gainof the APD.The avalanche process introduces excess noise because every photo-generated carrier does not undergo thesame multiplication.An APD is compact and immune to magnetic fields, requires low currents, is difficult to overload and hashigh quantum efficiency that can reach 90 %. The noise properties of an APD are affected bythat the APD is made of. Typicalthe materialssemiconductor materials used in the construction of low-noise APDsinclude silicon (Si), indium gallium arsenide (InGaAs) and germanium (Ge). More information concerningphotons and APD can be found in [Saleh and Teich, 1991] and [Meschede, 2004].